In June 1972, warm moist air from the Gulf of Mexico made it all the way to South Dakota. A cold front came down and 15 inches of rain fell on the Black Hills in just 6 hours. Now, the Pactola Dam was built in 1952 and it created flood control and a resevoir that made the region much more habitable. Ten miles away, Rapid City grew to nearly 50,000 and a large residential neighborhood sprung up. Trouble was, the neighborhood was situated in the flood plain. On This Date In 1972 after the torrential rains came, a bunch of debris clogged the spillway at the dam. The dam collapsed and water was sent rushing through the neighborhood and through Rapid City. Some 238 people lost their lives that night. Most of the homeowners had no insurance. Today the dam has been rebuilt. I suppose it was an earthen dam in 1972 as it is today. There’s one big difference. There is no residential community in the flood plain any more. Today it’s a golf course. It’s always amazing how when we look back at historical events that what seems like such an obviously stupid idea today was such a good idea back when it was first proposed. It often seems to take a tragedy to wake people from their slumber and remember one of US Grant’s favorite lines: “Man Proposes and God Disposes.”

Weather Bottom Line: This is an update to similar post earlier. The story is about the same though the SPC expanded the severe risk area markedly. I suspect this is in response to the divergence in model solutions as there is a consistency of a strong short rolling around to the north and then that secondary short moving out of the plains and into the region. But, its very difficult to nail down exactly where the second short will go and even the extent of the northern, stronger short. Guess here is that the enhanced humidity level, warm conditions and increased instability will lend itself to supporting strong storms wherever the kicker short comes through. Otherwise, there will be the risk of scattered convectively induced activity. While I’m still of the mind that the secondary short may be mainly south of our area, I have some concern of the extent of the instability of the northern short. Further, the secondary short may very a shade from my suspicions and so a variance of say 30 miles at initiation will make a lot of difference downstream. Tornadic threat would most likely be minimal as support conditions for such activity will probably be limited, but local conditions can always produce some monkeybusiness. Biggest threat will be for hail and strong winds dragged down from aloft.

…THERE IS A SLGT RISK OF SVR TSTMS FROM TX/OK/KS EWD INTO THE MID
OH/TN VALLEY REGION…

…SYNOPSIS…
A BROAD ZONE OF ENHANCED/WSWLY MID-LEVEL FLOW IS FORECAST TO
CONTINUE EAST OF THE ROCKIES THIS PERIOD…BETWEEN AN UPPER TROUGH
OVER CENTRAL CANADA AND A RIDGE CENTERED OVER THE WRN GULF OF
MEXICO. WITHIN THIS ZONE OF FASTER WSWLY FLOW…SEVERAL SHORT-WAVE
TROUGHS WILL MIGRATE ENEWD…CONTRIBUTING TO A BROAD ZONE OF
CONVECTIVE POTENTIAL FOCUSED INVOF A SURFACE FRONT FORECAST TO LIE
FROM THE OH VALLEY REGION WSWWD INTO THE SRN PLAINS THROUGH THE
PERIOD.

…KS/OK EWD INTO THE MID MS/LOWER OH/TN VALLEYS…
SOMEWHAT COMPLEX FORECAST THIS PERIOD FROM ERN KS/ERN OK EWD INTO
THE TN/OH VALLEY REGION…AS AT LEAST TWO EPISODES OF CONVECTION/MCS
SHOULD SHIFT EWD ACROSS ERN KS/MO AND INTO THE MID MS/LOWER OH/TN
VALLEY REGION THROUGH THE DAY ONE PERIOD. SEVERE POTENTIAL —
PRIMARILY IN THE FORM OF DAMAGING WINDS AND HAIL — WITH THE ONGOING
CONVECTION IS EXPECTED TO SPREAD ACROSS KY/TN AND VICINITY THROUGH
THE DAY…ALONG WITH THE POTENTIAL FOR NEW DEVELOPMENT NEAR THE
PERIPHERY OF ONGOING STORMS. MEANWHILE FARTHER W ACROSS SRN MO AND
INTO ERN KS/ERN OK…QUESTIONS EXIST REGARDING BOTH AIRMASS QUALITY
AND THE POTENTIAL FOR A MORE SWD/CONVECTIVELY-ALTERED LOCATION OF
THE SURFACE FRONT ACROSS THIS REGION.

ATTM…IT APPEARS THAT SOME HEATING/DESTABILIZATION MAY OCCUR ACROSS
SRN MO AND VICINITY…IN THE WAKE OF THE CONVECTIVE CLUSTER OR
CLUSTERS WHICH SHOULD BE SHIFTING EWD INTO THE MID MS/TN/OH VALLEYS.
NEW STORM DEVELOPMENT SHOULD OCCUR INVOF THE SURFACE BOUNDARY WITH
SOME WWD REDEVELOPMENT OF THE LOW-LEVEL JET LATER IN THE DAY WITHIN
THIS REGION OF POTENTIAL DESTABILIZATION. WITH MODERATELY-STRONG
DEEP-LAYER WIND FIELD…AMPLE INSTABILITY WOULD SUPPORT SEVERE
STORMS — WITH SUPERCELLS AND/OR LINEARLY-ORGANIZED CONVECTION
LIKELY. WHILE A FEW TORNADOES WOULD BE POSSIBLE NEAR THE FRONT IF
CELLULAR STORM MODE COULD BE MAINTAINED…MORE LIKELY POTENTIAL
WOULD BE FOR DAMAGING WINDS AND HAIL — ESPECIALLY WITH LIKELY
EVOLUTION INTO A CONVECTIVE CLUSTER/MCS THROUGH THE EVENING.

…N TX SWWD TOWARD THE TX BIG BEND REGION…
THE NAM AND GFS DIFFER IN TIMING OF FEATURES MOVING THROUGH MODERATE
WSWLY MID-LEVEL FLOW…AND THUS IN HANDLING OF THE SURFACE PATTERN
ACROSS TX. THE NAM — WITH A WEAK FEATURE PROGGED TO MOVE ACROSS W
TX THROUGH THE AFTERNOON — DEVELOPS A WEAK SURFACE LOW NEAR THE TX
BIG BEND…AND ASSOCIATED/WEAK SELYS AT THE SURFACE ACROSS PARTS OF
NRN AND WRN TX. MEANWHILE…THE GFS PROGS A STRONGER FEATURE TO
MOVE OUT OF NM AND INTO THE TX PANHANDLE/WRN OK BY LATE AFTERNOON.
THUS — THE GFS SURFACE LOW RESIDES OVER SWRN OK DURING THE
AFTERNOON…WITH MUCH MORE VEERED/SWLY SURFACE WINDS S OF THE LOW
OVER THE WRN HALF OF TX BEHIND AN EWD-MIXING DRYLINE.

THESE DIFFERENCES WOULD RESULT IN A MUCH GREATER SEVERE POTENTIAL IN
THE NAM SCENARIO…WITH MORE NUMEROUS STORMS DEVELOPING OVER
PORTIONS OF WRN TX WITHIN A MUCH MORE FAVORABLY-SHEARED ENVIRONMENT.
ATTM…WILL LEAN TOWARD THE GFS SOLUTION — WHICH WOULD BE FOR A
LESSER SEVERE THREAT WITH LESS NUMEROUS STORMS DEVELOPING FARTHER E.
WHILE A 15%/SLIGHT RISK FORECAST WILL BE MAINTAINED…WILL SHIFT
SEVERE PROBABILITIES SLIGHTLY WWD AS A HEDGE IN THE DIRECTION OF THE
NAM SCENARIO.

…MID-ATLANTIC REGION…
MODERATE DESTABILIZATION IS FORECAST ACROSS THE MID-ATLANTIC REGION
AHEAD OF A WEAK SURFACE TROUGH/CONVERGENCE AXIS EXPECTED ACROSS
VA/NC AND VICINITY. THOUGH DEEP-LAYER WIND FIELD WILL BE
MODEST…PULSE OR WEAK MULTICELL STORMS SHOULD DEVELOP INVOF THE
SURFACE TROUGH — ACCOMPANIED BY A LOW-END THREAT FOR HAIL/WIND.
CONVECTION AND ANY ASSOCIATED SEVERE POTENTIAL SHOULD GRADUALLY
DECREASE THROUGH THE EVENING.